1. Field of the Invention
The present invention relates to an underwater cutting pelletizer for use in producing resin pellets.
2. Description of the Related Art
As known well, an underwater cutting pelletizer for use in producing resin pellets is disposed on a downstream side of an extruder and is provided with a die plate, the die plate having nozzles through which molten resin (molten material to be processed) is extruded, and a cutter for cutting the resin extruded from the nozzles into pellets while rotating. In addition, there is provided a circulation box which covers the die plate and the cutter. The interior of the circulation box serves as a water chamber filled with circulating cooling water.
The water chamber is provided with an inlet portion for introducing cooling water into the interior of the water chamber. Cooling water supplied into the water chamber through the inlet portion not only cools the die plate and the cutter but also cools the pellets to a predetermined temperature and then flows out to the exterior from an outlet portion of the water chamber. At the same time, the cut pellets are also discharged to the exterior of the water chamber.
In such a water chamber it is very important for the cooling water to flow in accordance with a predetermined way of flowing. For example, it is necessary that the cooling water flow near the die plate and the cutter positively without stagnation. By so doing, not only the cutter and the die plate but also the cut pellets can be cooled to a predetermined temperature.
However, if the flow of the cooling water is not smooth and the cooling water stagnated within the water chamber, the cut pellets may stay for a long time within the water chamber or conversely may be discharged in a too short time from the water chamber. Thus, there is a possibility that the pellets cooling time may not coincide with a predetermined cooling time, with consequent deterioration in quality of the pellets. When the pellets stay for a long time within the water chamber, it is likely that the remaining pellets may strike against the cutter or the inner wall of the water chamber, or may stock each other, exerting a bad influence on the resin cutting work.
A technique for avoiding such an inconvenience is disclosed in Japanese Patent Laid-Open No. 2003-260706.
According to the technique disclosed in Japanese Patent Laid-Open No. 2003-260706, in order to make the temperature of cooling water which contacts a die plate uniform and thereby make the shape and size of the resulting pellets uniform, (i) at least three inlet portions are provided for introducing cooling water into a water chamber and (ii) an outlet portion for discharging the cooling water to the exterior is provided at a position deviated from extension lines of all the inlet portions.
The present inventors have performed a simulation of the flow of cooling water chamber in accordance with the technique disclosed in Japanese Patent Laid-Open No. 2003-260706. This simulation revealed the following points. The flow of cooling water is apt to stagnate in the vicinity of joined portions between the inlet portions and the water chamber and a joined portion between the outlet portion and the water chamber and pellets staying in the stagnation are not discharged to the exterior of the water chamber, further, complicated flows are formed within the water chamber by the flows of cooling water from the plural inlet portions and agitation of the cooling water caused by rotation of the cutter in the water chamber, resulting in occurrence of variations in the intra-water chamber retention time of cut pellets.
These phenomena give rise to a difference in pellets cooling time, with a great possibility of occurrence of variations in the quality, shape and size of pellets.
Moreover, since plural inlet portions are provided, a layout design and fabrication of cooling water supply pipes become complicated and as the case may be the flow of cooling water within the water chamber depends on the positional relation of plural inlet portions and outlet portion and the flow rate of cooling water. A lot of trials are needed for determining optimum values of such positional relation and flow rate.